The effect of aging on cognitive ability varies from one individual to the next. Recent studies suggest that one of the factors influencing individual differences in cognitive aging is the use of hormone replacement therapy (HRT). For example, recent findings from the Baltimore Longitudinal Study on Aging (BLSA) suggests that the use of estrogen replacement therapy (ERT) protects against age-associated declines in memory and against the development of Alzheimer's disease. One limitation of such studies is that the women who participated in them elected on their own to receive ERT. Research suggests that such women tend to be better educated and to receive better health care than women who do not receive ERT. The bias is termed the "healthy user effect."